This invention relates to a plated aluminum or aluminum alloy sheet which as improved spot weldability and which is suitable for use in the manufacture of automobile bodies.
Because of a low specific weight of 2.7 (which is about one-third that of iron), aluminum sheet including aluminum alloy sheet has begun to be employed in automobile bodies, particularly automobile hoods, for the purposes of saving weight and thereby reducing fuel consumption.
A major problem involved in the use of aluminum sheet in such applications is attributable to its spot weldability which is inferior to that of steel sheet conventionally used for automobile bodies.
Compared to steel, aluminum has significantly poorer heat generation efficiency in resistance welding such as spot welding since it is difficult to generate heat due to its low electrical resistivity, which is on the order of one-third to one-fourth that of steel, and the generated heat easily escapes due to its high thermal conductivity, which is on the order of 2 to 3 times that of steel. As a result, spot welding of aluminum sheets requires a current which is about four times as large as that required for spot welding of steel sheet.
Furthermore, a firm oxide film readily forms on the surface of aluminum or aluminum alloy, and this film is responsible for the formation of weld spots having inconsistent strength, resulting in poor reliability of spot welding.
Since a large current is passed, as described above, between the aluminum sheets and the electrodes of a spot welder, which are usually made of Cu or a Cu alloy, the surface of the electrodes tends to be rapidly contaminated with aluminum to form a brittle Cu--Al alloy. As a result, the service life of the electrodes (the number of weldable spots before re-grinding of the electrodes becomes necessary) is as small as between about 200 and about 300 spots, which is much smaller than the service life of 10,000 spots or more obtained with steel sheet.
Accordingly, there is a great need to improve the spot weldability of aluminum sheet, and various pretreatment methods have been attempted in the prior art for this purpose.
For example, a relatively simple pretreatment method known in the prior art is to remove the oxide film formed on the surface of aluminum sheet by grinding with Emery paper or a wire brush. This method is variable with respect to the extent that it can remove the oxide film, so it cannot improve the spot weldability to a desired level.
Another known method is to pretreat aluminum sheet by chemical conversion treatment such as phosphate chromating prior to spot welding. The method, however, does not produce a sufficient improvement in spot weldability. It is also proposed to improve spot weldability by removing the surface oxide film by the washing action of arc. However, this pretreatment method is impractical since the incorporation of the pretreatment method prior to spot welding in a manufacturing line of automobile bodies greatly increases the equipment costs.
Japanese Patent Application Laid-Open No. 53-6252(1978) and Japanese Patent Publication No. 54-41550(1979) disclose interposing a thin zinc film at the weld interface, i.e., between two aluminum sheets to be spot welded, in order to improve spot weldability. The thin zinc film is either a zinc foil inserted in the interface or a zinc coating or plating formed on one or both of the aluminum sheets.
According to that method, it is expected that the efficiency of heat generation can be improved by the zinc film interposed between the aluminum sheets. However, the improvement depends on the thickness of the zinc film, and a sufficient effect cannot be obtained with a zinc film having a thickness of about 10 .mu.m or less, which approximately corresponds to a weight of about 70 g/m.sup.2 or less. Furthermore, due to the fact that the melting point of zinc (420.degree. C.) is lower than that of aluminum (660.degree. C.), the thin zinc film is melted prior to melting of aluminum sheets upon passage of welding current and the molten zinc extends over the weld interface, resulting in the diffusion of current. Therefore, that method requires an increased current compared to a conventional spot welding method for aluminum sheets, and the heat generated at the weld interface between the aluminum sheets and electrodes is increased, thereby leading to a diminished service life of the electrodes.
Aluminum also suffers from rather poor press-formability. Aluminum sheet has a local deformability lower than that of steel sheet and is apt to fracture when a concentrated strain is imposed thereon. In addition, the surface sliding properties of aluminum sheet are inferior to those of steel sheet, and this fact is also responsible for the poor press-formability of aluminum sheet.